The purpose of the primary study, Prophylaxis Against Early Adrenal Insufficiency to Prevent Chronic Lung Disease (CLD), is to determine whether low-dose hydrocortisone therapy in the first two weeks of life in ventilated extremely low birth weight (ELBW) infants at increased risk for adrenal insufficiency will improve survival without CLD and without adverse effects. The purpose of this ancillary study is to determine whether the intervention of low-dose hydrocortisone therapy will improve cardiovascular function, stabilize blood pressure, and reduce use of inotropic agents in the study population of the primary study. Based on the findings from previous relevant research, the primary study will test the hypotheses that early adrenal insufficiency in ELBW infants results in chronic lung disease, and that early low-dose cortisol replacement therapy during the first two weeks of life would correct this deficiency and decrease the incidence of chronic lung disease, and that cortisol replacement therapy during that time would also prevent symptoms of acute adrenal insufficiency, such as hypotension, hyponatremia, hyperkalemia, and delayed weight loss. The ancillary study will test the hypothesis that early low-dose hydrocortisone replacement therapy in ventilated ELBW infants at increased risk for adrenal insufficiency will improve cardiovascular function, stabilize blood pressure, and reduce use of inotropic agents.